The synthesis of light-harvesting rods can be achieved through use of porphyrinic building blocks. A major goal in the development of porphyrin or chlorin building blocks for application in light-harvesting systems is to fine-tune their photophysical and electrochemical properties. Tuning has been achieved with porphyrins by incorporating various substituents at the meso as well as β-positions, introduction of different core metals, etc. (Li, F. et al., J. Mater. Chem. 1997, 7, 1245-1262; Yang, S. I. et al., J. Porphyrins Phthalocyanines 1999, 3, 117-147; Yang, S. I. et al., J. Am. Chem. Soc. 1999, 121, 4008-4018). Chlorins have two considerable advantages for light-harvesting applications in comparison with porphyrins: (1) the extinction coefficient of the long-wavelength absorption band is greater than that of porphyrins (Chang, C. K. et al., Proc. Natl. Acad. Sci. USA, 1981, 78, 2652-2656; Stolzenberg, A. M. et al., J. Am. Chem. Soc. 1981, 103, 4763-4778), and (2) chlorins are linear oscillators whereas metalloporphyrins are planar oscillators. These differences make chlorins superior to porphyrins for use in photosynthetic systems. Chlorin building blocks are quite attractive for applications in light-harvesting systems. The important photochemical properties of chlorins have motivated the development of a number of new syntheses of these green pigments in the past few years (Taniguchi, M. et al., J. Org. Chem. 2001, 66, 7342-7354; Jacobi, P. A. et al., Org. Lett. 2001, 3, 831-834; Montforts, F.-P. et al., Angew. Chem. Int. Ed. 2000, 39, 599-601; Shea, K. M. et al., Tetrahedron. 2000, 56, 3139-3144; Burns, D. H. et al., Chem. Commun. 2000, 299-300; Strachan, J.-P. et al., J. Org. Chem. 2000, 65, 3160-3172; Balasubramanian, T. et al., J. Org. Chem. 2000, 65, 7919-7929; Krattinger, B. et al., J. Org. Chem. 1999, 1857-1867; Johnson, C. K. et al., Tetrahedron Lett. 1998, 39, 4619-4622; Mironov, A. F. et al., J. Chem. Soc. Perkin Trans. l 1998, 3601-3608).
Chlorin dyads are of particular interest as benchmarks for assessing the extent of electronic communication and the rates and efficiency of energy transfer between chlorins. Nevertheless, relatively little attention had been paid to the synthesis and comparative studies of covalently linked chlorin dyads, because chlorins have limited stability and the synthesis of chlorin building blocks has been quite challenging. A number of chlorin dyads with various linkages and configurations have been prepared. These chlorin dyads are categorized in the following groups: (1) chlorins with electron acceptors for electron-transfer studies (Tkachenko, N. V. et al., J. Am. Chem. Soc. 1999, 121, 9378-9387; Malinen, P. K. et al., Liebigs Ann. 1997, 1801-1804; Abel, Y. et al., Tetrahedron Lett. 1997, 38, 1745-1748; Zheng, G. et al., Chem. Commun. 1999 2469-2470; Lindsey, J. S. et al., J. Am. Chem. Soc. 1988, 110, 3610-3621), (2) chlorins possessing fused aromatic rings (Kozyrev, A. N. et al., Tetrahedron 2000, 56, 3353-3364; Silva, A. M. G. et al., Tetrahedron Lett. 2000, 41, 3065-3068; Johnson, C. K. et al., Tetrahedron Lett. 1998, 39, 4753-4756; Krattinger, B. et al., Chem. Commun. 1998, 757-756), (3) chlorin-porphyrin dyads for energy-transfer studies (Faustino, M. A. et al., Photochem. Photobiol. 2000, 72, 217-225; Zheng, G. et al., Tetrahedron Lett. 1997, 38, 2409-2412; Wasielewski, M. R. et al., Solar Energy Materials and Solar Cells 1995, 38, 127-134; Johnson, D. G. et al., J. Am. Chem. Soc. 1993, 115, 5692-5701; Zenkevich, E. I. et al., J. Luminescence 1997, 75, 229-244), (4) chlorins with accessory pigments (Kutzki, O. et al., Helv. Chim. Acta, 2000, 83, 2231-2245; Wedel, M. et al., J. Org. Chem. 2001, 1681-1687; Vicente, M. G. H. et al., Chem. Commun. 1998, 2355-2356), (5) chlorin-chlorin dimers (Arnold, D. P. et al., Tetrahedron 2001, 57, 1335-1345; Zheng, G. et al., J. Org. Chem. 2000, 65, 543-557; Osuka, A. et al., Heterocycles 1997, 44, 165-168; Jaquinod, L. et al., Angew. Chem. Int. Ed. Engl. 1996, 35, 1013-1016), (6) oxochlorin-containing dyads (Kessel, D. et al., Photochem. Photobiol. 1991, 53, 475-479; Osuka, A. et al., Bull. Chem. Soc. Jpn. 1995, 68, 262-276; Osuka, A. et al., J. Am. Chem. Soc. 1996, 118, 155-168).
The conversion of chlorins to oxochlorins has little effect on the spectral properties but renders the macrocycle more resistant to oxidation (Chang, C. K. et al., J. Am. Chem. Soc. 1986, 108, 1352-1354; Stolzenberg, A. M. et al., Inorg. Chem. 1986, 25, 983-991; Zaleski, J. M. et al., J. Phys. Chem. 1993, 97, 13206-13215; Osuka, A. et al., Bull. Chem. Soc. Jpn. 1993, 66, 3837-3839). The general approach for forming an oxochlorin employs treatment of a β-substituted porphyrin with a suitable oxidizing agent such as hydrogen peroxide followed by acid-catalyzed pinacol rearrangement of the resulting diol (Inhoffen, von H. et al., Liebigs Ann. Chem. 1969, 725, 167-176; Bonnet, R. et al., J. Chem. Soc. (C) 1969, 564-570; Chang, C. K. Biochemistry 1980, 19, 1971-1976; Chang, C. K. et al., J. Org. Chem. 1986, 51, 2134-2137) or OsO4 (Chang, C. K. et al., J. Org. Chem. 1985, 50, 4989-4991; Chang, C. K. et al., Heterocyclic Chem. 1985, 22, 1739-1741; Chang, C. K. et al., J. Biol. Chem. 1986, 261, 8593-8596; Brückner, C. et al., Tetrahedron Lett. 1995, 36, 3295-3298; Brückner,-C. et al., Tetrahedron Lett. 1995, 36, 9425-9428; Pandey, R. K. et al., J. Org. Chem. 1997, 62, 1463-1472). This procedure works well with those porphyrins that give rise to only one oxochlorin isomer. Although this method takes advantage of porphyrins as readily available starting materials, multiple oxochlorin isomers are obtained from porphyrins that possess any of the following structural features: (1) at least one of the four pyrrole rings bears substituents that are different from the others, in which case multiple products are formed due to the reaction of OsO4 at different sites; (2) different substituents (e.g., methyl/ethyl) are present at the two β-positions of the pyrrole ring that is attacked by OsO4, in which case pinacol rearrangement gives multiple products; and (3) the presence of meso-substituents that cause the symmetry of the porphyrin to be less than four-fold, in which case pinacol rearrangement of the diol gives multiple products. Oxochlorins of the latter type have been incorporated in a few multi-pigment arrays for studies of energy and electron transfer (Osuka, A. et al., Bull. Chem. Soc. Jpn. 1995, 68, 262-276; Osuka, A. et al., J. Am. Chem. Soc. 1996, 118, 155-168).